The present invention relates, in general, to frequency standards and, more particularly, to frequency standards using hydrogen masers to maintain a stable standard frequency that is free of drift.
An atomic frequency standard is a device having a basic resonant system derived from an atomic or molecular specie experiencing a transition between two well-defined energy levels of the atom or molecule. The transition occurs in a reasonably convenient domain of the electromagnetic spectrum, the microwave region. The transition is employed as a highly stable frequency reference to which the frequency of a voltage-controlled crystal oscillator (VCXO) can be electronically locked. Thus, the high stability and relative insensitivity associated with an atomic reference frequency is thereby transferred to the VCXO.
Hydrogen, cesium and rubidium frequency standards can provide atomic-controlled oscillators in which the frequency of usually a 5 MHz or 10 MHz quartz crystal oscillator is controlled by means of a physics package and associated electronics that are devoted to maintaining that assigned output on a very long-term, exceedingly accurate and stable basis. By properly slaving the quartz crystal oscillator to the frequency of the atomic transition, the tendency of the quartz crystal to exhibit drifting due to aging and other inherent as well as environmental effects is markedly suppressed.
In a hydrogen maser frequency standard, hydrogen is customarily stored under pressure in a container and is passed through and purified by a hot palladium-silver alloy diffusion barrier that provides control of the hydrogen flux and subsequent beam intensity. The hydrogen, in molecular form, is introduced into an electrical discharge defined by a dissociation chamber which dissociates diatomic hydrogen into atomic hydrogen. Thereafter, the resultant hydrogen atoms emerge in a low-pressure region, are collimated into a beam and directed or focused through a state selection device, usually having a passageway of a few millimeters in diameter. The state selection device may be a hexapole or quadrapole magnet having a magnetic flux density at the pole tips of generally about 0.7 T or more. The state selection device generates an inhomogeneous magnetic field and is designed in such a manner so as to withdraw atoms in the lower hyperfine energy state and allows those hydrogen atoms to pass having the upper hyperfine state into a high Q-cavity resonator located in a microwave field region. In the resonator, the atoms of hydrogen undergo interaction with a microwave field at a resonance frequency of about 1.4 GHz.
The cavity resonator into which the selected atoms of hydrogen are directed is surrounded by magnetic shields and is provided with an inner solenoid that creates a weak, substantially uniform magnetic field which is applied to the microwave field region in order to separate the different sublevels of the hyperfine state to insure that transitions occur only between levels where the Zeeman effect is quadratic.
The cavity resonator is designed to allow extremely long interaction times of atoms with the microwave field by coating the walls of the resonator with special compositions, generally a fluorocarbon or the like to reduce atom perturbations through wall collisions. The interaction with the microwave field induces the hydrogen atoms in the selected energy state to make a transition to the lower state, thus radiating energy of a precise frequency to the field. In an active maser system, maser oscillations are self-sustaining when the atom density in the cavity resonator is such that the resultant induced transitions radiate enough coherent energy to more than offset cavity losses.
The hydrogen that departs from the cavity resonator may be readily removed by a vacuum system, generally by means of a high speed, getter pump along with a titanium sputter ion pump, both of which are generally housed in separate vacuum chambers. The getter materials that may be used for such a pump include Zr-Al, Zr-C and Zr-V alloys.
Although the general conditions recited above relate to an active maser mode, viz., using the maser principle itself in which there is coherent stimulated emission of the radiation within a suitable resonant structure, there is another mode, the so-called passive mode which may be employed for standard frequency generators. In the passive mode, an ensemble of particles (i.e. atoms or molecules) undergoing the desired quantum transition and an auxiliary source of radiation (slave oscillator) is employed to produce the transitions which occur when the frequency of the radiation is near the atomic resonance frequency. Thus, in a passive frequency standard, the atomic resonance has to be probed by an electromagnetic signal at the proper frequency which is produced by an auxiliary frequency generator whereas in an active frequency standard there is self-sustaining oscillation at the atomic resonance frequency.
In 1920 R. W. Wood was the first to employ a device to generate atomic hydrogen. Since then others have employed various means to produce atomic hydrogen including aerodynamic shock tubes, thermal dissociation means as well as radio frequency discharge devices. This latter technique has proven most convenient and, therefore, has been a preferred technique used in the art. In general, a r.f. discharge is employed in which external electrodes are placed proximate a glass dissociation chamber to produce a r.f. field capacitively or inductively coupled to a gaseous discharge or plasma generated within the chamber. The resultant hydrogen atoms emerge from the chamber into a low-pressure region through an orifice to define a stream of atomic hydrogen in the form of a beam.
The stream of hydrogen atoms leaving the dissociation chamber includes atoms at several energy states, and hydrogen atoms that are not at the desired hyperfine energy state are focused out of the beam generally by a state selector magnet through which the stream of hydrogen atoms is directed.
The hydrogen atoms in the selected hyperfine energy state are then directed into a microwave cavity that is resonant at about the hydrogen resonance frequency of about 1.4 GHz. To obtain effective operation of a passive maser, it is desirable that the cavity be maintained at the hydrogen resonance frequency and that the microwave energy introduced into the cavity be maintained at hydrogen resonance frequency. Unfortunately, environmental perturbations, such as changes in pressure, aging of cavity coatings, deformation of the cavity due to strain or shock or temperature changes, and cavity pulling detune the cavity. In addition, the electronic components used to provide the microwave energy to the cavity are also subject to aging and environmental changes which produce a change or drift in frequency.
To correct for cavity detuning and frequency changes and to eliminate drift and provide a stable standard frequency source, the microwave energy introduced into the microwave cavity has been modulated by two frequencies. One frequency modulation is adapted to probe the microwave cavity for the effects of detuning influences and to permit the generation of a corrective action for cavity detuning, and another much smaller frequency modulation is adopted to permit generation of an error signal for deviations of frequency of the microwave radiation introduced into the cavity from hydrogen resonance frequency and maintenance of the frequency of the microwave radiation at the hydrogen resonance frequency. Such corrective methods and apparatuses are disclosed in U.S. Pat. Nos. 3,792,368 and 4,122,408 and in "A Small Passively Operated Hydrogen Maser", D. A. Howe, F. L. Walls, Howard E. Bell, and H. Hellwig, published in Proc. 33rd Annual Symposium on Frequency Control, (1979), pp. 554-592.
As yet no compact, relatively inexpensive, self-contained, passive hydrogen maser frequency standard has been commercially available which may be reliably and easily used in non-laboratory environments. Such a practical, commercial passive hydrogen maser frequency standard is believed to have been inhibited by a number of factors which have militated against the effective and reliable use of the hydrogen hyperfine transition in a passive maser.
The beam-producing device is a critical component for microwave amplification, in the maser and similar desires. As can be appreciated the efficiency of producing a stream of atoms, such as hydrogen, determines to a large measure the available flux of the beam and, accordingly, the stability of the maser.
It follows from the above considerations that proper alignment is essential in placing a stream of atomic species in a favorable directional mode. For an atomic maser, it is essential that a beam of atoms be directed through a state selector and into the cavity resonator. Although a number of dissociation devices with fixed configurations have been employed, none have actually provided adequate focusing or alignment means offered directly from the discharge source, a shortcoming that has heretofore been neglected. Admittedly, the path is a straight one as the stream must pass through a state selector having a magnetic field as well as through a cavity tube leading into the cavity resonator of the maser. However, there is a need for fine angular adjustment through the magnetic field in order to optimize the effectiveness of the maser operation. Heretofore no practical steering or alignment device has been available for this critical aspect of precisely adjusting and directing atomic beams into cavity resonators and the like.
In addition, the microwave cavity has a tendency to oscillate in undesirable modes of oscillation; and such undesirable oscillations adversely affect the frequency of the transition of the hydrogen atoms between their hyperfine energy states, as measured by the circuit means coupled to the microwave cavity.
Furthermore, because the circuit means to detect variations of cavity energy due to variations in the microwave cavity and provide a corrective action to the cavity for aging and environmental perturbations uses relatively large frequency modulation, e.g., 12 KHz; and because the circuit means to detect variation of cavity energy due to variations in the frequency of the introduced microwave energy from the hydrogen resonance frequency and provide a corrective action to the VCXO uses a small frequency modulation, e.g., 12 Hz, there is a tendency for one of the phase modulation sidebands due to the small frequency modulation, particularly one of the sidebands separated from the carrier by an even number of modulation intervals, e.g., the second upper or second lower sideband, to become locked with the hydrogen resonance energy in the microwave cavity, resulting in instability, unpredictable frequency changes, inaccuracy, and lack of repeatability.